In making cardboard boxes, a wooden die is prepared that is used to crease the cardboard so that it is folded accurately. A kerf is made in the plywood that serves to hold a steel rule that imprints the fold lines on the cardboard when the die is placed in a press. If the kerf is not accurately cut, the rule will not be held or will not be in the right position and the cardboard will not be creased properly. This problem is exacerbated by the fact the wooden dies are often very large.
The laser has long been used for cutting the knifing grooves in flat and rotary dieboards. For many years the technology was limited to the use of a 500 watt CO2 lasers with fixed distances from the laser to the focusing lens to cut plywood into desired shapes for use as dieboards. The key processing parameters were the control of top and bottom kerf dimensions, kerf verticality or perpendicularity to the board surface, and kerf walls that remained within known tolerances of being straight.
From the nature of the laser cutting process, the dieboards were placed on an open faced enclosure or "tub" which would catch scrap material, collect the smoke and fumes for proper exhaust, and provide for laser safety by eliminating the exposure of the operators to laser radiation. As a consequence of this construction, the bottom of the dieboard is not accessible to the operator during the cutting process, and a "direct" measurement of the bottom kerf width is not possible. Operators have developed an indirect procedure for determining the correctness of this dimension by inserting a blade or "rule" into the kerf and "feeling" the resistance to insertion and withdrawal.
As technology has advanced to higher power lasers and moving optic systems, throughput has increased dramatically, and this increased productivity has called for more automatic systems. One obvious impediment to higher productivity is the inability to determine the bottom kerf width while cutting so that in-process adjustments can be made. While the top kerf width is readily controlled by focusing techniques, the bottom kerf width is dependent on a "burning" process which can be affected by a number of variables inherent in the plywood being cut, the laser, the motion system, the process environment and system programming.
In the current process, test cuts are made in the plywood in the X and Y directions and taper gauges are inserted to measure the top width of kerf. Based upon this measurement, an appropriately sized rule is inserted into the kerf to obtain a "feel" for tightness and an estimate of the bottom width. Laser and motion system parameters are consequently adjusted and additional test cuts are made until the estimated bottom widths are correct. Once actual cuts are made, the dimensions may or may not be periodically checked in a similar fashion over the entire board length depending upon its size, material, required accuracy, and operator sensitivity.
As is apparent from the above description, the current measuring process is imprecise and is performed after the material is fully cut, resulting in increased scrap. In addition, the offline measurement of the plywood and subsequent adjustment of the cutting process results in significant and costly machine down-time. Therefore, there is a need for a system for measuring the kerf of a laser cut plywood dieboard which provides accurate in-process measurement and adjustment of the system variables such that both scrap and machine down-time are reduced.